1. Field of the invention
This invention relates broadly to endoscopes and laparoscopes. More particularly, this invention relates to objective lens systems for endoscopes and laparoscopes.
2. State of the Art
Endoscopes are optical systems which are well known in the art to permit observation of otherwise inaccessible areas within the human body without the use of excessive surgery. Besides minimizing the invasiveness of the surgery, endoscopes provide the advantage that, depending on the optical resolution of the endoscope, the taking of biopsy samples for later laboratory analysis may be unnecessary where direct endoscopic optical diagnosis is sufficient. Endoscopes which are specifically used to examine the peritoneal cavity (pertaining to the abdominal and pelvic cavities) of a patient are referred to as laparoscopes.
Structurally, endoscopes generally include an airtight and waterproof elongated tube having a distal end with an objective lens which is placed within the body cavity being examined and a proximal end with an eyepiece for viewing. The elongated tube includes a relay lens system between the objective lens and the eyepiece. The function of the objective lens is to form an image at the distal end of the tube. The function of the relay lens system is to take the image formed at the distal end of the tube and to transmit the image to the proximal end of the tube. The function of the eyepiece is to permit an observer to observe the image which is transmitted to the proximal end of the tube.
The objective lens is usually a relatively short focal length, wide angle lens situated at the distal end of the relay lens system behind a water and air proof "window" of a very durable optical material. The objective lens is typically made of at least two or more spherical lens elements which incorporate corrective means for the various chromatic and geometric (spherical, field curvature, astigmatic, coma) aberrations inherent in lenses. It is very important that the objective lens form an accurate and bright an image of the observed area. The image formed by the objective lens will be reimaged by the relay lens system several times before it is viewed at the eyepiece of the endoscope. Since reimaging will unavoidably degrade the image to some extent, the objective lens must produce the best image possible.
Such a high quality image is not easily produced due to several imaging problems present within conventional lens systems. The first problem relates to the image brightness of an optical system. The image brightness of an optical system is affected by the shape, composition and size of its lens elements. The smaller the focal length to diameter ratio of an optical system, the faster the system will be and the brighter the transmitted image will appear. This ratio is also referred to as the f/number. In laparoscopic optical systems, the f/number is usually defined by the elements following the objective, such as the relay system. The image brightness is also affected by transmission losses caused by light absorption and scattering within the lens elements. An example of absorption loss can be illustrated by using a lens with a slight amber cast to form an image from an object. In such a lens, any blue light originating from the original object would be absorbed by the amber colored lens, thus causing a chromatic distortion in the formed image as well as a reduction in brightness. Scattering loss occurs as the result of the presence of incompletely or improperly polished lens elements in the optical system. In such a case, the lens elements may not absorb the light directly, but instead scatter it out of the intended ray paths to be absorbed by the walls of the system or simply lost. Scattering can also be the cause of poor contrast in an image as a result of the flooding of the image plane with unwanted and out of focus light.
A second common problem associated with conventional lenses is that of image aberrations, such as third order sphericals, comas and astigmatisms, all of which reduce the sharpness of the formed image. Additionally, field curvature may be such that the image cannot viewed in its entirety from any one angle, that is be accommodated over an angular extent all at the same time by a normal eye, although the eye could focus on different areas in the image at different times.
A third problem found in conventional lens system is distortion. Most conventional lenses exhibit a small amount of radial distortion. Radial distortion occurs when the radial image scale varies as view angle goes from the center of the field of view to the edge of the field of view. Radial distortion is the cause for the "fish eye" effect produced by extremely wide field lenses.
Another problem in designing an objective lens system is that a very durable as well as air/water proof element is needed as part of the objective lens system to act as a window between the area being observed and the inside of the endoscope. This element must usually have a flat surface facing the direction of area to be observed in order to avoid any change in optical power when the endoscope is immersed in a fluid.
Known objective lens designs are often costly to manufacture due to the materials used and the difficulties of accurately fabricating these materials to optical tolerances. Specifically, any non-polymeric lens element is very costly to manufacture and may not necessarily be adequately temperature resistant or air and water resistant. In addition, the prior art fails to disclose temperature resistant objective lens elements which could be used in an autoclavable endoscope design.